Update spiders.rst
Update spiders.rst

Added note to allowed_domain attribute with an example explaining what goes in the list
Ryezhang/scrapy,elacuesta/scrapy,umrashrf/scrapy,ArturGaspar/scrapy,kmike/scrapy,starrify/scrapy,pawelmhm/scrapy,Ryezhang/scrapy,eLRuLL/scrapy,dangra/scrapy,finfish/scrapy,pablohoffman/scrapy,dangra/scrapy,wujuguang/scrapy,kmike/scrapy,finfish/scrapy,scrapy/scrapy,kmike/scrapy,elacuesta/scrapy,starrify/scrapy,ssteo/scrapy,Ryezhang/scrapy,pawelmhm/scrapy,eLRuLL/scrapy,ssteo/scrapy,scrapy/scrapy,pablohoffman/scrapy,dangra/scrapy,wujuguang/scrapy,scrapy/scrapy,pablohoffman/scrapy,starrify/scrapy,finfish/scrapy,wujuguang/scrapy,umrashrf/scrapy,Parlin-Galanodel/scrapy,Parlin-Galanodel/scrapy,ssteo/scrapy,pawelmhm/scrapy,ArturGaspar/scrapy,elacuesta/scrapy,Parlin-Galanodel/scrapy,ArturGaspar/scrapy,eLRuLL/scrapy,umrashrf/scrapy