Replace deprecated <tt> tag

[ahornace]

<tt> tag is not HTML5 compliant (https://www.w3schools.com/tags/tag_tt.asp).

Thanks for review :)

[coveralls]

Pull Request Test Coverage Report for Build 2397

• 0 of 3 (0.0%) changed or added relevant lines in 2 files are covered.
• 22 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.01%) to 73.214%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/org/opensolaris/opengrok/web/DirectoryListing.java	0	1	0.0%
src/org/opensolaris/opengrok/search/Results.java	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/org/opensolaris/opengrok/history/FileHistoryCache.java	1	73.52%
src/org/opensolaris/opengrok/history/GitRepository.java	2	62.04%
src/org/opensolaris/opengrok/configuration/RuntimeEnvironment.java	2	69.26%
src/org/opensolaris/opengrok/analysis/AnalyzerGuru.java	2	85.27%
src/org/opensolaris/opengrok/history/HistoryGuru.java	2	73.75%
src/org/opensolaris/opengrok/history/RepositoryFactory.java	3	85.42%
src/org/opensolaris/opengrok/history/SCCSRepository.java	3	78.91%
src/org/opensolaris/opengrok/history/BitKeeperRepository.java	3	82.95%
src/org/opensolaris/opengrok/history/MercurialRepository.java	4	81.72%

Totals
Change from base Build 2394:	-0.01%
Covered Lines:	30818
Relevant Lines:	42093

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💛 - Coveralls