ORIGIN Grammar Typo in Some Documentation

[segaloco]

Hello, just reporting that lines 492-494 of ld/ld.texi contain examples of 
MEMORY ORIGIN of the form:

    ORIGIN : offset

Where the grammar is:

origin_spec:
        ORIGIN '=' mustbe_exp
                {
                  region->origin_exp = $3; 
                }
        ;

Seems like hoops to raise a pull request, someone else can take the glory.

- Matt G.