cmd/compile: fix Compiler Directives typo

Change-Id: I098b3c627c2142affd4e800a2c1f37e00f3775c5
Reviewed-on: https://go-review.googlesource.com/c/140777Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/doc.go

@@ -146,7 +146,7 @@ directive can skip over a directive like any other comment.
 // will report positions in the original input to the generator.
 /*
 The line directive is an historical special case; all other directives are of the form
-//go:name and must start at the begnning of a line, indicating that the directive is defined
+//go:name and must start at the beginning of a line, indicating that the directive is defined
 by the Go toolchain.
 
 	//go:noescape