fix typos

CollectionIdentifiers.md

@@ -1,6 +1,6 @@
 # USI Collection Identifiers
 
-Universal Spectrum Identifier (USI) for Mass Spectrometry has as its second field a collection identifier.
+The Universal Spectrum Identifier (USI) for Mass Spectrometry has as its second field a collection identifier.
 The specification itself does not prescribe the complete list of allowable collection identifiers. Rather,
 the list is maintained here at GitHub and is extensible in an open manner without changing the specification.
 
@@ -10,11 +10,11 @@ the list is maintained here at GitHub and is extensible in an open manner withou
 (http://proteomecentral.proteomexchange.org/dataset/PXD017269). The PXDnnnnnn SHOULD be used for all proteomics
 mass spectrometry datasets that have been registered at ProteomeXchange.
 
-2. MSVnnnnnnnnn - A collection identifier for a datasets contained at the MassIVE repsositorym such as MSV000078556. This type of 
+2. MSVnnnnnnnnn - A collection identifier for a datasets contained at the MassIVE repsository, such as MSV000078556. This type of 
 identifier SHOULD only be used when there is not a corresponding PXDnnnnnn identifier for the dataset, as is often the case
 for metabolomics datasets.
 
-3. RPXDnnnnnn - a ProteomeXchange identifier for a reprocessed dataset of the form RPXDnnnnnn, such as RPXD006668
+3. RPXDnnnnnn - A ProteomeXchange identifier for a reprocessed dataset of the form RPXDnnnnnn, such as RPXD006668
 (http://proteomecentral.proteomexchange.org/dataset/RPXD006668). The RPXDnnnnnn SHOULD ONLY be used when there is no
 corresponding PXDnnnnnn identifier available (as in the case where an RPXDnnnnnn dataset derives from a non-ProteomeXchange
 dataset.) The PXDnnnnnn identifier should always be used when possible.