spec: clarify zero value for complex types

The enumeration of numeric types missed the complex types.
Clarify by removing the explicit enumeration and referring
to numeric types instead.

Fixes #21579.

Change-Id: If36c2421f8501eeec82a07f442ac2e16a35927ba
Reviewed-on: https://go-review.googlesource.com/58491
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

@@ -6158,7 +6158,7 @@ <h3 id="The_zero_value">The zero value</h3>
 and no explicit initialization is provided, the variable or value is
 given a default value.  Each element of such a variable or value is
 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
-<code>0</code> for integers, <code>0.0</code> for floats, <code>""</code>
+<code>0</code> for numeric types, <code>""</code>
 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
 This initialization is done recursively, so for instance each element of an
 array of structs will have its fields zeroed if no value is specified.