runtime: fix documentation typo for gostartcall

This CL is a simple doc typo fix, uncovered while reviewing the go-wasm
port.

Change-Id: I0fce915c341aaaea3a7cc365819abbc5f2c468c3
Reviewed-on: https://go-review.googlesource.com/80715Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_x86.go

@@ -11,7 +11,7 @@ import (
 	"unsafe"
 )
 
-// adjust Gobuf as it if executed a call to fn with context ctxt
+// adjust Gobuf as if it executed a call to fn with context ctxt
 // and then did an immediate gosave.
 func gostartcall(buf *gobuf, fn, ctxt unsafe.Pointer) {
 	sp := buf.sp