gui: Clarify block height label

Summary:
> Prefer "block height" instead of "number of blocks".

This is a backport of [[bitcoin-core/gui#40 | core-gui#40]]

Test Plan:
`ninja && src/qt/bitcoin-qt`
Menu Window > Information

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10130

Author: hebasto

src/qt/forms/debugwindow.ui

@@ -290,7 +290,7 @@
        <item row="11" column="0">
         <widget class="QLabel" name="label_3">
          <property name="text">
-          <string>Current number of blocks</string>
+          <string>Current block height</string>
          </property>
         </widget>
        </item>