typo in bson_writer_get_length.rst
typo in bson_writer_get_length.rst

rcsanchez97/libbson,mongodb/libbson,rcsanchez97/libbson,mapr/libbson,bjori/libbson,ajdavis/libbson,mongodb/libbson,rcsanchez97/libbson,mongodb/libbson,bjori/libbson,ajdavis/libbson,ajdavis/libbson,mapr/libbson,mapr/libbson,bjori/libbson