write "libbson" lower-case in installing.rst
write "libbson" lower-case in installing.rst

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