DOC: DeviceAtlas documentation typo fix.

The USE_PCRE syntax was incorrect. No backport needed.
diff --git a/doc/DeviceAtlas-device-detection.txt b/doc/DeviceAtlas-device-detection.txt
index ccafaa0..bb4e369 100644
--- a/doc/DeviceAtlas-device-detection.txt
+++ b/doc/DeviceAtlas-device-detection.txt
@@ -5,7 +5,7 @@
 the API source code from https://deviceatlas.com/deviceatlas-haproxy-module.
 The build supports the USE_PCRE and USE_PCRE2 options. Once extracted :
 
-    $ make TARGET=<target> USE_PCRE (or USE_PCRE2=1) USE_DEVICEATLAS=1 DEVICEATLAS_SRC=<path to the API root folder>
+    $ make TARGET=<target> USE_PCRE=1 (or USE_PCRE2=1) USE_DEVICEATLAS=1 DEVICEATLAS_SRC=<path to the API root folder>
 
 Optionally DEVICEATLAS_INC and DEVICEATLAS_LIB may be set to override the path
 to the include files and libraries respectively if they're not in the source