perlapi - HeVAL operates on HEs, not HVs

The _perlapi_ example for `HeVAL` is misleading/confusing.

Instead of:

    SV *foo= HeVAL(hv);
    HeVAL(hv)= sv;

It should read:

    SV *foo= HeVAL(he);
    HeVAL(he)= sv;

Rationale: `hv` is the variable name commonly given to a `HV*`,
`he` is a logical name to give a `HE*`. `HeVAL` operates on the latter.

Author: richardleach

hv.h

@@ -249,8 +249,8 @@ Returns the value slot (type C<SV*>)
 stored in the hash entry.  Can be assigned
 to.
 
-  SV *foo= HeVAL(hv);
-  HeVAL(hv)= sv;
+  SV *foo= HeVAL(he);
+  HeVAL(he)= sv;
 
 
 =for apidoc Am|U32|HeHASH|HE* he