BUG: Fix header formatting in README.rst

Underline of a title must extend at least as far as the right edge of the title text (https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#sections). Currently, `twine check` in the `Build, test, package` workflow fails due to this error.

Author: Yu-AnChen

README.rst

@@ -1,5 +1,5 @@
 ITKFixedPointInverseDisplacementField
-=================================
+=====================================
 .. image:: https://github.com/InsightSoftwareConsortium/ITKFixedPointInverseDisplacementField/workflows/Build,%20test,%20package/badge.svg
 
 ITKFixedPointInverseDisplacementField takes a displacement field as input and