doc/effective_go: make function signature match function body

In the Slices section of Effective Go, the os package's File.Read
function is used as an example. Unfortunately the function signature
does not match the function's code in the example, nor the os package's
documentation. This change updates the function signature to match
the os package and the pre-existing function code.

Change-Id: Iae9f30c898d3a1ff8d47558ca104dfb3ff07112c
Reviewed-on: https://go-review.googlesource.com/9845Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -1382,7 +1382,7 @@ limit of how much data to read.  Here is the signature of the
 <code>os</code>:
 </p>
 <pre>
-func (file *File) Read(buf []byte) (n int, err error)
+func (f *File) Read(buf []byte) (n int, err error)
 </pre>
 <p>
 The method returns the number of bytes read and an error value, if