spec: shorten heading for complex, real, imag

In the current TOC it sticks out quite a bit.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5705053

doc/go_spec.html

@@ -4894,7 +4894,7 @@ a no-op. Calling <code>delete</code> with a nil map causes a
 </p>
 
 
-<h3 id="Complex_numbers">Assembling and disassembling complex numbers</h3>
+<h3 id="Complex_numbers">Manipulating complex numbers</h3>
 
 <p>
 Three functions assemble and disassemble complex numbers.