Change header "APP_VERSION" to "APP VERSION"

Update following review comment:
- https://github.com/helm/helm/pull/5538#pullrequestreview-221803355

Signed-off-by: Martin Hickey <martin.hickey@ie.ibm.com>

cmd/helm/history.go

@@ -35,7 +35,7 @@ configures the maximum length of the revision list returned.
 The historical release set is printed as a formatted table, e.g:
 
     $ helm history angry-bird --max=4
-    REVISION    UPDATED                     STATUS          CHART             APP_VERSION     DESCRIPTION
+    REVISION    UPDATED                     STATUS          CHART             APP VERSION     DESCRIPTION
     1           Mon Oct 3 10:15:13 2016     superseded      alpine-0.1.0      1.0             Initial install
     2           Mon Oct 3 10:15:13 2016     superseded      alpine-0.1.0      1.0             Upgraded successfully
     3           Mon Oct 3 10:15:13 2016     superseded      alpine-0.1.0      1.0             Rolled back to 2

cmd/helm/testdata/output/history-limit.txt

@@ -1,3 +1,3 @@
-REVISION	UPDATED                      	STATUS    	CHART           	APP_VERSION	DESCRIPTION 
+REVISION	UPDATED                      	STATUS    	CHART           	APP VERSION	DESCRIPTION 
 3       	1977-09-02 22:04:05 +0000 UTC	superseded	foo-0.1.0-beta.1	1.0        	Release mock
 4       	1977-09-02 22:04:05 +0000 UTC	deployed  	foo-0.1.0-beta.1	1.0        	Release mock

cmd/helm/testdata/output/history.txt

@@ -1,4 +1,4 @@
-REVISION	UPDATED                      	STATUS    	CHART           	APP_VERSION	DESCRIPTION 
+REVISION	UPDATED                      	STATUS    	CHART           	APP VERSION	DESCRIPTION 
 1       	1977-09-02 22:04:05 +0000 UTC	superseded	foo-0.1.0-beta.1	1.0        	Release mock
 2       	1977-09-02 22:04:05 +0000 UTC	superseded	foo-0.1.0-beta.1	1.0        	Release mock
 3       	1977-09-02 22:04:05 +0000 UTC	superseded	foo-0.1.0-beta.1	1.0        	Release mock

pkg/action/history.go

@@ -165,7 +165,7 @@ func getReleaseHistory(rls []*release.Release) (history releaseHistory) {
 func formatAsTable(releases releaseHistory) []byte {
 	tbl := uitable.New()
 
-	tbl.AddRow("REVISION", "UPDATED", "STATUS", "CHART", "APP_VERSION", "DESCRIPTION")
+	tbl.AddRow("REVISION", "UPDATED", "STATUS", "CHART", "APP VERSION", "DESCRIPTION")
 	for i := 0; i <= len(releases)-1; i++ {
 		r := releases[i]
 		tbl.AddRow(r.Revision, r.Updated, r.Status, r.Chart, r.AppVersion, r.Description)