[C++/Format] Change name of Fixed Width Binary to Fixed *Size* Binary for consistency

[asfimport]

Based on discussion in ARROW-725

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow 0.3 release (blocks)

_{Note: This issue was originally created as ARROW-733. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #473

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 473
#473