Updates PMID detection regex

Why are these changes being introduced:

* The regex was missing PMIDs with no space between the `PMID:`
  and the indentifier, such as `PMID:35648703`

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/TCO-42

How does this address that need:

* Updates regex to allow, but not require, a single whitespace character
  between `pmid:` and the indentifier

app/models/standard_identifiers.rb

@@ -33,7 +33,7 @@ def term_patterns
     {
       isbn: /\b(ISBN-*(1[03])* *(: ){0,1})*(([0-9Xx][- ]*){13}|([0-9Xx][- ]*){10})\b/,
       issn: /\b[0-9]{4}-[0-9]{3}[0-9xX]\b/,
-      pmid: /\b((pmid|PMID): (\d{7,8}))\b/,
+      pmid: /\b((pmid|PMID):\s?(\d{7,8}))\b/,
       doi: %r{\b10\.(\d+\.*)+/(([^\s.])+\.*)+\b}
     }
   end

test/models/standard_identifiers_test.rb

@@ -157,7 +157,7 @@ class StandardIdentifiersTest < ActiveSupport::TestCase
   end
 
   test 'pmid examples' do
-    samples = ['PMID: 35648703', 'pmid: 1234567']
+    samples = ['PMID: 35648703', 'pmid: 1234567', 'PMID:35648703']
 
     samples.each do |pmid|
       actual = StandardIdentifiers.new(pmid).identifiers