spec: More precise wording in section on function calls.

A caller is not always in a function.

For example, a call can appear in top level declarations.

e.g. var x = f()

Change-Id: I29c4c3b7663249434fb2b8a6d0003267c77268cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/290849
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Feb 2, 2021",
+	"Subtitle": "Version of Feb 10, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -3446,7 +3446,7 @@ <h3 id="Calls">Calls</h3>
 After they are evaluated, the parameters of the call are passed by value to the function
 and the called function begins execution.
 The return parameters of the function are passed by value
-back to the calling function when the function returns.
+back to the caller when the function returns.
 </p>
 
 <p>