text/template/parse: fix a comment around the assign operator

Fix a comment that misrepresented the Assign operator (=).

Rename: colon-equals -> equals.

Change-Id: I405b8acfb0bcd1b176a91a95f9bfb61a4e85815f
GitHub-Last-Rev: aec0bf594c63d7b015f88f97f9953ade976817a4
GitHub-Pull-Request: golang/go#26112
Reviewed-on: https://go-review.googlesource.com/121416Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/text/template/parse/lex.go

@@ -42,7 +42,7 @@ const (
 	itemChar                         // printable ASCII character; grab bag for comma etc.
 	itemCharConstant                 // character constant
 	itemComplex                      // complex constant (1+2i); imaginary is just a number
-	itemAssign                       // colon-equals ('=') introducing an assignment
+	itemAssign                       // equals ('=') introducing an assignment
 	itemDeclare                      // colon-equals (':=') introducing a declaration
 	itemEOF
 	itemField      // alphanumeric identifier starting with '.'