README: fix typo

13 jobs for update_readme in 21 minutes and 4 seconds (queued for 1 second)
Status Name Job ID Coverage
  Compilation
passed cmp:cc7-docker #6220068
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passed cmp:cc7-gcc #6220067
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passed cmp:cc7-llvm #6220069
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passed cmp:mac1014-clang #6220070
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passed cmp:slc6-gcc #6220065
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passed cmp:slc6-llvm #6220066
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  Testing
passed tst:telescope #6220071
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  Formatting
passed fmt:cc7-llvm-lint #6220075
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00:09:44

passed fmt:centos7-llvm-format #6220074
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passed fmt:slc6-llvm-format #6220072
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passed fmt:slc6-llvm-lint #6220073
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  Documentation
passed cmp:doxygen #6220076
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passed cmp:usermanual #6220077
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