fix docbook install failure

rename e_command page to e.xml. This has a case insensitive naming
conflict with E.xml on some platforms but big E is slotted to be
replaced by a drawing mode, so move it's man page out of the way
temporarily to bigE.xml

doc/docbook/system/mann/CMakeLists.txt

@@ -1,7 +1,7 @@
 set(mann_EN
   3ptarb.xml
   B.xml
-  E.xml
+  bigE.xml
   M.xml
   Z.xml
   adc.xml
@@ -66,7 +66,7 @@ set(mann_EN
   dm.xml
   draw.xml
   dup.xml
-  e_command.xml
+  e.xml
   eac.xml
   echo.xml
   edcodes.xml

doc/docbook/system/mann/E.xml → doc/docbook/system/mann/bigE.xml

@@ -1,14 +1,14 @@
-<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="E">
+<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="bigE">
 
   <refmeta>
-    <refentrytitle>E</refentrytitle>
+    <refentrytitle>bigE</refentrytitle>
     <manvolnum>nged</manvolnum>
     <refmiscinfo class="source">BRL-CAD</refmiscinfo>
     <refmiscinfo class="manual">BRL-CAD User Commands</refmiscinfo>
   </refmeta>
 
   <refnamediv xml:id="name">
-    <refname>E</refname>
+    <refname>bigE</refname>
     <refpurpose>
       Display <emphasis>objects</emphasis> in an evaluated form.
     </refpurpose>