doc: removing biblio keywords (that are not shown anyway) to please new Sphinx version

doc/bib_manual.bib

@@ -1341,15 +1341,7 @@ year = "2013",
 note = "",
 issn = "1047-8477",
 doi = "http://dx.doi.org/10.1016/j.jsb.2013.10.006",
-author = "Pilar Cossio and Gerhard Hummer",
-keywords = "Electron microscopy",
-keywords = "Cryo-EM",
-keywords = "Bayesian inference",
-keywords = "Ensemble refinement",
-keywords = "Validation",
-keywords = "Disorder",
-keywords = "GroEL",
-keywords = "\{ESCRT\} "
+author = "Pilar Cossio and Gerhard Hummer"
 }