Fix inconsistency of input field placeholder text capitalization

The board search input field of the "Select Other Board and Port" dialog uses placeholder text to explain the usage of
the field to the user.

All other placeholder text in the IDE's UI uses sentence case. This specific placeholder was the exception, using
unpleasant caps lock instead.

The inconsistency is resolved by changing the placeholder text to the standard sentence case.

Author: per1234

Diff for: arduino-ide-extension/src/browser/boards/boards-config.tsx

@@ -308,7 +308,7 @@ export class BoardsConfig extends React.Component<
             className="theia-input"
             placeholder={nls.localize(
               'arduino/board/searchBoard',
-              'SEARCH BOARD'
+              'Search board'
             )}
             onChange={this.updateBoards}
             ref={this.focusNodeSet}

Diff for: i18n/en.json

@@ -27,7 +27,7 @@
       "portLabel": "Port: {0}",
       "programmer": "Programmer",
       "reselectLater": "Reselect later",
-      "searchBoard": "SEARCH BOARD",
+      "searchBoard": "Search board",
       "selectBoard": "Select Board",
       "selectBoardForInfo": "Please select a board to obtain board info.",
       "selectPortForInfo": "Please select a port to obtain board info.",