Update README.md

[briochemc]

Minor fixes to the ReadMe:

• "block-block-banded" -> "block-banded"
• changed the BlockBandedMatrix description to explicitly state that the layout of the blocks is banded
• removed an extraneous closing parenthesis in the code snippet for BandedBlockBandedMatrix

[codecov]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   84.43%   84.43%           
=======================================
  Files          10       10           
  Lines         925      925           
=======================================
  Hits          781      781           
  Misses        144      144           

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[dlfivefifty]

Thanks!